A fuel assembly of a known design comprises a plurality of parallel fuel rods and control rod guide tubes, wherein fuel rods and guide tubes are retained into an elongated bundle in predetermined mutual positions by means of a number of spacer grids, referred to below simply as spacers, arranged across the bundle. Top and bottom nozzles are arranged at the ends of the fuel assembly and attached to the control rod guide tubes, which extend somewhat above and below the ends of the fuel rods. In known fuel assemblies each control rod guide tube has been fixed to the bottom nozzle by means of a screw extending through the bottom nozzle and threaded into a corresponding thread in an end plug fixed to the control rod guide tube. This, in turn, has tended to give rise to an axial rotation of the control rod guide tube. Such a rotation may cause a built-in torsional stress in the guide tube, which in turn may lead to the fuel bundle, after irradiation, becoming spirally twisted. This may eventually lead to handling problems when transferring fuel in the core, which, of course, entails a serious drawback.